Programmable material handling robots have been in use for some time. Typically such robots include an articulated (i.e. jointed) arm or mobile (i.e. rail mounted) arm with an end effector mounted at the end of the arm. The end effector is the part of the robot that directly interfaces with its environment. For material handling robots this generally includes a gripper of some sort—either with two or more opposing grip pickup arms which close together to grip a load, or single grip arm which closes against a fixed piece to grip a load.
Material handling robots used in conjunction with automated or semi-automated production lines provide numerous advantages in efficiency, accuracy and endurance that make them preferable to having personnel perform such operations manually. Work force head counts may be markedly reduced by replacing line workers with robots for repetitive tasks and providing a small cadre of technical staff to support the robots. Material handling robots are also extremely useful in situations where precise and/or highly repeatable placement is required.
Replacing personnel with robots reduces injuries in jobs heavy lifting or awkward movements, especially repetitive movements, and in potentially dangerous or harsh environments which might threaten employee health with prolonged exposure. Examples of such harsh environments include production and material handling lines for dry concrete powder, silica, powder detergents, salt, fertilizers, aggregate, sand, alumina, mortar, grout, clay and other environments where abrasive products or harsh chemicals are manipulated and moved. Although the bags of concrete powder, for example, are closed, handling bags still generates clouds of concrete dust. This dust is unhealthy to breathe and may irritate mucous tissues, such that workers handling the bags are required to wear dust masks or respirators, safety goggles and gloves for protection. This would be an ideal situation to replace humans with robots.
However, the dust generated is also extremely abrasive and corrosive due to the hardness of the dust and frequently the high alkalinity or acidity of the materials which breaks down lubricants and can corrode aluminum, brass, copper and non-stainless steel parts, as well as many plastic materials. Robot components are frequently fabricated from aluminium to save weight. Brass fittings are frequently used in pneumatic systems due to resistance to corrosion from moisture.
Many material handling robot end effectors utilize pneumatic cylinder actuators to grasp, manipulate and release materials. These cylinder actuators have a very short operating life in such environments. The primary failure modes are the wear surfaces between the cylinder seal—cylinder bore (i.e. internal to the cylinder) and the cylinder rod—wiper seal (i.e. external), and the wear surfaces at the clevis joints where the cylinder rods connect to a load (e.g. a robot finger, clutch plate, etc.). Cylinder failure occurs through abrasion of the seals or internal cylinder walls which causes binding and allows air pressure to leak out, thereby seizing the cylinders. Guided cylinders include guide rod bearings which are vulnerable to fouling, erosion and failure, and which thereby cause the operating cylinder to seize. Bushing wear from abrasive materials opens tolerances, accelerating wear, and can cause moving parts to misalign or impact with each other, leading to robot crashes and/or fatigue failures. Joints fail through fouling of lubricants and erosion of bearing surfaces and binding from dust buildup. Higher operating speeds, with consequently greater friction and higher loads on wear surfaces, accelerate component failure.
Abrasion may be compounded by corrosion caused by the compounds within the dust. For example, concrete dust is highly alkaline, which is destructive to cylinder and end effector components made from aluminum. Some plastic fittings and bearing surfaces may also become brittle and crack. Lubricants tend to break down in such environments. Dust accumulation may also tend to have high hydroscopic cross section which draws in moisture from the atmosphere thereby exacerbating the problems. The lubricants required by air cylinders and associated wear parts actually attract dust to the friction surfaces that the lubricants are intended to protect, thereby exacerbating the wear problems discussed above.
The problems resulting from the accelerated component failure in harsh environment include excessive maintenance labor, replacement parts costs, production downtime, and product loss from mishandling by robots with end effectors which fail to operate properly.
Past solutions to harsh environment problems have focused primarily on making wear surfaces with more wear-resistant and/or corrosion resistant materials such as stainless steel or hardened carbon steel, adding air purge systems to prevent excessive dust accumulation on of equipment, installing wear components within flexible sleeves, bellows or boots, and adding sacrificial wear plates or sleeves to high friction points which are less expensive to replace than the entire component. These solutions are not ideal, add significant expense due to higher fabrication costs (e.g. stainless steel and carbide steel components), and still may impose high maintenance requirements even if failure timing becomes more predictable.
Additional problems with handling bags of powder or loose granular materials such as aggregate or seed is that such bags lack rigidity. In order to increase the robot operating speeds means must be provided to clamp the bags to prevent them from shifting in the end effector during movement. Prior solutions have focused on using pushdown plates operated by a pneumatic cylinder to maintain pressure on a bag after gripping with an end effector, but the pushdown plate cylinders suffer the same drawbacks as described above in harsh environments.
The inventors have found that many of the problems of harsh operating environments are better solved by designing end effectors to substantially reduce the number of wear surfaces by using alternative, non-traditional actuation methods, and linking components to achieve adequate range of motion for end effector components but with shorter stroke lengths of the pneumatic actuators. The inventors have found that end effectors constructed using airbags rather than cylinders eliminates many wear surfaces entirely. The use of airbags rather than cylinders eliminates entirely the wear surfaces between the cylinder seal and chamber and the cylinder rod and wiper. The use of pusher plates rather than clevis joints also eliminates a substantial number of wear surfaces. Airbags can be fabricated from chemically resistant rubber or polymer materials which won't be damaged by sliding friction as frequently happens to coated cylinder components, Airbags do not require lubrication which attracts dust to wear surfaces.
Airbags may also be used to hold down bags after pick up by the end effector to prevent shifting during movement and ensure stacking efficiency. Airbags are less susceptible to fouling and failure, as described above, and are also less likely to tear or damage a powder bag than a contact plate. Airbags will tend to distribute pressure more evenly over the contact surface because the airbag is more malleable compared to a plate. Providing a firm clamp or grip on bags of powder also permits alternative stacking geometries which can be more stable. For example, an end effector may position itself skewed or offset toward a selected end of a bag prior to pickup, such that it grips the bag firmly but the opposing end hangs free to a certain degree. When unloading the powder bag, the free-hanging end will contact first, so the robot can then release the gripped end and lay the bag down in a controlled fashion. This control also facilitates stacking the bags in an overlapping crisscross pattern for greater overall stability of the stack.
Pneumatic cylinders are also more susceptible to misalignment and consequent jamming—even under normal industrial operating conditions. Airbags don't jam in place, as cylinders often due in harsh environments. There are no internal moving parts which may become cockeyed due to unbalanced forces—the pressure within the bag is transmitted equally throughout the volume of the bag and over its entire surface area. Cylinder-based systems accommodated such misalignments by adding joints (such as clevis plates or universal joints) to provide flex at connection points. However, adding joints or adding bending axes to existing joint connections, merely compounds the problems discussed above relating to wear surfaces. Airbags will continue to operate reliably even with sore misalignment without need for additional joint connections or additional bend axes on existing joints—the bag itself can accommodate much of the out-of-axis force created by minor misalignments.
Another problem associated with material handling robots is that of positioning a load prior to pickup by the robot end effector. Correct load positioning is a particular problem for soft goods such as bags of powder coming off a conveyor because they deform easily during handling. Robots which utilize an overhead pickup orientation, such as when transferring from a conveyor to stack bags of powder onto shipping pallets in an alternating orientation for stability, must ensure the bags are properly positioned in the end effector grip. Positioning is critical first to ensure the pick up occurs, but also to prevent unbalanced forces on the robot during rapid movement, and to ensure precise load placement during drop off. Positioning loads prior to pick up is particularly critical when the loads are bags of abrasive or chemically corrosive materials, because dropping or damaging the bags will release the product and exacerbate the environmental problems already prevalent in the area. Existing solutions generally utilize some sort of pusher cycled by a pneumatic cylinder. Although positioning is not a problem unique to harsh environments, harsh environments create the same problems as described above relating to pneumatic cylinders. Airbag operated positioning devices operate more reliably in such environments, which in turns leads to fewer damaged bags releasing powder which contributes to the harsh environment—a virtuous cycle.
Yet another problem associated with material handling robots operating in harsh environments is that actuators may fail singly, or actuator degradation may occur even without complete failure. Thus, in an end effector having left and right-side operators, degradation may cause one side to operate more slowly, or may cause one side to be more “sticky” than the other, such that the load is gripped properly by one side, but poorly or not at all by the other side, leading to inaccurate stacking and placement, or damaged loads which release yet more harmful dust and particulates. This sort of degraded operation may not be detected by sensors in time to prevent load failures, or requires additional sensors which increase costs and maintenance—and add failure points to the system. In such cases, it would be useful to incorporate a synchronizer linking the operators so that they open and close at the same speed, and so when one side “sticks” force is transferred from the opposing side to un-stick the operator, freeze both in place.
Airbags are not new. However, while airbags have been used for lifting loads, the inventors are not aware of any use of airbags to operate intricate mechanical systems such as a robot end effector. The use of airbags require a differing design approach which has not previously been fully appreciated in the art.